Wall arrangement for vapor generator



March 9, 1965 R. A. KANE 3,172,396

WALL ARRANGEMENT FOR VAPOR GENERATOR Filed March 25, 1963 F'IC3-| 6 75 2 Sheets-Sheet 1 I IHIHHIIIHIIIHIIIIHII lllllllll HIIIIIIIIIIIIIIHIHI r March 9, 1965 R. A. KANE WALL ARRANGEMENT FOR VAPOR GENERATOR Filed March 25, 1963 2 Sheets-Sheet 2 United States Patent 3,172,396 WALL ARRANGEMENT FOR VAPGR GENERATOR Robert A. Kane, Hazardville, Conn, assignor to Comhustion Engineering, Inc, Windsor, Conn, a corporation of Delaware Filed Mar. 25, 1963, Ser. No. 267,516 7 Claims. (Cl. 122-406) This invention relates generally to high-capacity, highpressure vapor generators such as employed in the utility industry and has specific relation to such a vapor generator of the forced throughlow type operating at supercritical pressures and temperatures.

In accordance with the present invention there is provided an improved forced through-flow supercritical vapor generator organized so that welded wall construction is employed throughout the furnace and the gas pass which extends from the furnace with the tubular members that line the inner surface of the side walls of the furnace and the gas pass being welded together to form a continuous, rigid, interconnected fluid-tight surface that extends through the furnace on into and through the gas pass.

The supercritieal vapor generator of the invention includes an elongated furnace from which there extends, in conventional fashion, a gas pass with this gas pass having a horizontal portion that leads laterally from the furnace outlet at the upper portion of the rear Wall of the furnace and with this horizontal portion being connected with a downwardly extending portion which contains various convection surface or heat exchange sections of the vapor generator and which is provided at its lower region with a suitable outlet through which the combustion gases pass to an air heater or other equipment. The Walls of the furnace are lined with vertically extending tubes in sideby-side relation with these tubes being in parallel flow relation and with adjacent tubes being Welded together throughout the height of the furnace to form an imperforate, fluid-tight inner lining for the furnace walls. The side walls of the gas pass are co-planar with the side walls of the furnace and in effect form extensions of the side walls and the side walls of the horizontal portion of the gas pass as well as the side walls and the front and rear Walls of the vertical portion of the gas pass are also lined with vertically extending tubes which are in side-by-side relation with adjacent tubes being welded together throughout the height of the gas pass wall with which they are associated.

The through-flow circuit of the supercritical forced through-flow vapor generator includes a number of heat exchange sections which are connected in series flow relation with the feed pump forcing the through-flow or working fluid through this circuit and the heat exchange sections at supercritical pressure. Among the heat exchange sections that make up the through-flow circuit are the furnace wall tubes and the tubes that line the gas pass walls. The arrangement is such that temperature varia tions both transversely and longitudinally of the furnace and gas pass walls are maintained at such values as to permit the welded construction referred to Without developing excessive stresses. This result is achieved by arranging the furnace wall and the gas pass Wall tubes in immediate successive flow relation with the flow therethrough being in the same direction and in connecting across or superimposing upon the portion of the throughflow circuit which includes these wall tubes a recirculating system to increase the flow through this portion of the through-flow circuit at low loads and during startup. With this organization the through-flow is forced in parallel fiow relation up through the furnace Wall tubes and directly after traversing these tubes it is forced, also "ice in parallel flow relation, up through the gas pass wall tubes. The recirculating system is effective to insure that the flow through the high heat absorption sections of the through-flow circuit, and in accordance with the invention, through the gas pass wall tubes as well as the furnace wall tubes, does not fall below a certain value which for example may be 6% or 70% or more of the through-flow provided at maximum load. This has the effect of greatly stabilizing the temperature within the walls of the furnace and the gas pass, permitting the use of larger diameter tubes so that flow through the tubes is not influenced by deposits and scale to the extent that it would be with small diameter tubes. With the recirculating system the temperature difference in the tubes from top to bottom of the furnace as well as transversely of the furnace, particularly at the lower loads, is only a small portion of what it would be without the recirculating system. By maintaining this temperature difference at a lower value than would otherwise be the case, the stability of flow through the tubes is greatly improved and there is provided a more uniform temperature distribution throughout the tubes that make up the inner lining of the walls with consequent decrease in stresses and particularly cyclic stresses incident to load changes and startup and shutdown.

Accordingly it is an object of the present invention to provide an improved forced through-flow supercritical vapor generator.

It is a further object of the invention to provide such an improved generator wherein welded Wall construction is provided with the stresses therein being maintained at a relatively low value.

A still further object of the invention is to provide such an improved vapor generator wherein the furnace side walls and the gas pass side walls form a continuous, rigid welded structure with the gas pass side walls being Welded to the furnace side walls.

Other features and advantages will be apparent from the specification and claims, and from the accompanying drawing which illustrate a preferred embodiment of the invention and in which:

FIG. 1 is a vertical sectional view, somewhat diagrammatic in nature, of a forced through-flow supercritical vapor generator embodying the present invention;

FIG. 2 is a fragmentary view of the upper portion of the vapor generator of the invention being also a vertical section but showing the side wall of the furnace rather than the center or partition wall as shown in FIG. 1;

FIG. 3 is a transverse sectional View of the furnace of the vapor generator taken generally along line 3-3 of FIG. 1;

FIG. 4 is a detailed fragmentary sectional view of a number of the tubes lining the walls of the furnace and the gas pass and,

FIG. 5 is a diagram in the nature of a flow sheet, showing the interconnection of and the direction of the flow of the working medium through the tubes that make up the center Wall and the outer wall of the furnace andv the tubes that make up the Walls of the gas pass.

Considering now the illustrative and preferred embodiment of the invention depicted in the drawings the vapor generator includes the furnace 10 which is partitioned into two sections or chambers 12 and 14 by the partition or center wall to, each of which may be fired in any desired manner as by the tangentially arranged sets of burners 13 and 15. The furnace is of rectangular transverse configuration being bounded by the front wall 18,

rear wall 2% and side Walls 22. At the upper region erally at 24, being rectangular in transverse section, extending from this outlet and having the horizontal portion 26 which leads to the vertical portion 28. The side walls of the gas pass 24 are co-planar with and generally form extensions of the side walls 2?. of the furnace.

The vapor generator is of the forced, once-through flow type with the working medium being forced through the through-flow circuit of the vapor generator by means of the feed pump 30, which forces the working medium through the economizer 32 at supercritical pressure. The outlet of the economizer communicates with mixing vessel 34 through the conduit 36 and from this vessel 34 the fiuid flows down through the conduit 38 in which is connected the recirculation pump 40, with this conduit 33 connecting with the lower or inlet header 42 at the bottom of the partition or center wall 11.6. This wall 16 is comprised of single pass vertically extending tubes that are in side-by-side relation and that are welded together throughout the height of the wall. The through-flow passes in parallel relation through these tubes, identified as 44, and into the outlet header 46 at the upper end of this wall. 'From this outlet header 46 the through-flow is conveyed via the conduit 48 to the lower end of the furnace where it enters the headers 50 one of which is disposed beneath each of the furnace walls, i.e. front wall 18, rear wall 20 and side walls 22.

'Each of these walls is lined at its inner surface with vertically extending tubes 52 with each of these tubes making a single pass of the furnace and having their inlet ends connected with the header associated withthe particular wall with which the tube is associated. The tubes 52 that extend upward along the front wall 18 are bent at the upper end of the furnace to extend along the inner surface of the furnace roof to the plane of the back wall of the furnace and are there bent to connect with the distribution header 54-. The tubes 52 that line the side walls 22 are connected at their outlets with one of the headers 56, there being one such header for each side Wall, and these headers are in turn connected with the distribution header 54 through the conduit 58. The tubes 52 that extend upward along and line the innersurface of the rear wall 20 of the furnace are bent to conform with the inwardly projecting nose baffle at the upper region ofthe furnace and are then bent to extend along the floor of the horizontal portion 26 of the gas pass to approximately the midpoint of this horizontal portion, as shown in FIGS. 1 and 2, with these tubes thenbeing bent to extend vertically across the gas pass to the roof thereof and then extending back toward the front of the furnace to line the roof of the gas pass and finally being connected with the distribution header 54. When cross- The through-flow is in parallel relation through all of the tubes that line the furnace wall with the how progressing upwardly through the tubes from the inlet headers 50,

being finally received in the distribution header 54. From this header 54 the through-flow is conveyed to inlet headers 69 disposed beneath the side walls of the horizontal portion 26 of the gas pass, with there being one such header beneath each of the side walls of the gas pass, and to ring header 62 disposed at the lower end of the vertical portion 28 of the gas pass. It will be appreciated that the gas pass is rectangular in transverse section and that this ring header 62 is also rectangular having a portion disposed adjacent each of the walls of the gas pass. The header 54 is connected with these headers 60 and 62 through the downcomers or distribution conduits 65.

Extending up from each of the inlet headers 60 is: a

. 4. group of tubes identified as 64, which make up the heat exchan e section 66 and which line the two opposed side walls of the horizontal portion 26 of the gas pass. Extending up from the ring header 62 are the tubes 68 which line the four walls of the vertical portion of the gas pass; The tubes68 lining the front wall it? of this vertical portion 28 of the gas pass extend upwardly therealong and are then bent to extend along the fioor of the horizontal portion 26 of the gas pass to approximately the midpoint of this horizontal portion where they are bent to extend vertically across the gas pass to the roof thereof and whereat they are then bent, as shown, to extend rearwardly along the roofand are finally connected with the header 7%. Here again, it will bis-appreciated that the portion ofthe tubes which extend across the horizontal portion 26 of the gas pass, are bent out of alignment so as to provide openings for the free flow of combustion gases through the gas pass. The tubes 64 and 68 which line the side walls of the gas pass are connected at their upper ends with the header 72, there being one such header disposed above each of the side walls, with these headers in turn being connected with the header 7% through the conduit '74. The tubes, 68 which extend up wardly along the rear wall 76 of the gas pass are connected directly withthe header 7%. Adjacent tubes which line the horizontal portion 26 and the vertical portion 28 of the gas pass, i.e., tubes '64 and 68 are welded together throughout'the height of the gas pass with which the tubes are associated so that they preferably provide a fluid-tight and imperforate inner lining for the gas pass. Additionally, the tubes 64 which make up the heat exchange section 66 and which line the side walls of the horizontal portion 26 of the gas pass are welded to the tubes 68 where the horizontal gas pass portion meets the vertical gas pass portion and also are welded to the tubes 52 of the side walls of the furnace where this heat exchange portion 66 meets and joins with the furnace. Thus there is a continuous and rigid side wall structure formed throughout the furnace and the gas pass eliminating the need for expansion joints and special seals as required hereinbefore. Each of the tubes 64 and 68 make only a single pass or traverse of the gas pass.

After traversing the vertically extending tubes that line the gas pass 24 the through-flow is conveyed from the header 70 by means of the conduit 78 through the various superheater heat exchange sections which may include, in series flow relation, the superheater section 86, the superheater section 82 and finally the finishing superheater section 84, after traversal of which the supercritical vapor is heated to its desired temperature and from which it is conveyed to a turbine in conventional fashion. From the turbine the vapor may be returned to the generator for reheating being passed first through the low temperature superheater section 86 and then through the high. temperature superheater section 88 after which it may again be returned to the turbine in conventional fashion. The exhaust from the turbine is conveyed to the usual condenser and then through the feedwater system and finally to thefeed pump 3!) for repassage through the through-flow circuit of the vapor generator.

The particular circuit of the vapor generator wherein the through flow is conveyed up through the tubes that line the furnace walls in parallel flow relation and then up through the tubes that line the sides of the horizontal gas pass portion and the walls of the vertical gas pass portion, also in parallel relation (as best illustrated diagrammatically in FIG. 5), provides the most favorable temperature relation for the continuous welding of the wall tubes together as disclosed to provide a rigid construction while maintaining minimum stresses By having the flow in the same direction, temperature dif-- ferences are maintained at a minimum between adjacent furnace and gas pass wall tubes thatare, welded together at the juncture of the furnace and gas pass. The upflow of fluid in the tubes tends to prevent flow instability during startup and low load which otherwise may develop. In addition to this circuit arrangement contributing to maintaining stresses at a minimum, the stresses are maintained at'an optimum low value so as to make it possible to employ such a continuous welded wall construction by means of the recirculation system which is superimposed upon the through-flow circuit. This system includes the recirculation conduit 86 which has its inlet connected with the header 7tl an'd its outlet connected with the mixing vessel '34 with there being provided a suitable check valve and shut-off valve identified generally as 88 in this recirculation conduit. This conduit 86 together with'the pump 40 comprise a recirculation system which is effective to recirculatefluid through a portion of the through-flow circuit of the vapor generator, including a portion of highest heat absorption, so as to insure that adequate flow is provided through the tubes of this portion of the circuit so that they will not be overheated during startup and at low loads.

In accordance with the invention the recirculation system is connected across a portion of the through-flow circuit which includes the'tubes that make up the center wall, the tubes that line the outer furnace walls and the tubes that'line the walls of the gas pass, as most clearly illustrated'in FIG. 5. The recirculation system is operative to insure that the flow through this portion of the through-fiow circuit of the vapor generator does not decrease below a predetermined value. It is preferred that the recirculation pump be of the constant speed type and that it float on the line so its operation is automatic, as explained in detail in U.S. Patent 3,135,252 issued June 2, 1954, to W. W. Schroedter. Illustrative of the operation of the recirculation system it may automatically become effective to provide a recirculation of fluid for example between 60 and 70% of maximum load and insure that the flow through the portion of the through-flow circuit on which the recirculation system is superimposed does not fall below this value.

With the circuit arrangement of the invention and the recirculation system the temperature variation in the tubes of the continuous welded wall structure will not be sufiiciently great so as to set up excessive stresses which will tend, as a result of numerous startups of the unit, to cause fatigue failure.

In the through-flow circuit of the supercritical vapor generator of the invention the relation of the heat exchange sections is such that the transition zone is located in the furnace walls. Since this is the zone of highest specific heat throughout the circuit the most uniform temperature of the furnace wall tubes and accordingly the least stress will result.

There is shown in FIG. 4 an illustration of how the tubes may be welded together and in this illustration there is shown three of the tubes 52 that line the side wall of the furnace and two of the tubes 64 that line the horizontal portion 26 of the gas pass. There is positioned intermediate each of these tubes a web 66 which is welded at its opposite ends to the adjacent tubes. It will be appreciated that since the heat absorption in the gas pass 24 and particularly the vertical portion 28 of this gas pass is considerably less intense than at locations further upstream and in the furnace t0, the tubes that line the walls of the vertical portion 28 may be spaced further apart such that the web disposed in between adjacent tubes will be substantially larger than that employed in the furnace and in at least the upstream portion of the horizontal portion 26 of the gas pass. The tubes 68 of the rear gas pass may be provided with peg fins (rectangular fins) on diametrically opposite sides and these fins of adjacent tubes abutted and welded together to provide the rigid welded construction. With this construction this gas pass portion will have to be skin cased for fluid tightness but because of the welded construction slip joints will be eliminated.

Adequate protection of the tubes 44 of the center wall is assured for the reason that the total flow area provided by these tubes for the through-flow is substantially less than the total flow area provided by the tubes $2; lining the Walls of the vapor generator. Since the circuit is designed and the recirculation circuit superimposed on the through-flow circuit assures that adequate flow is provided through the tubes 52 so that they are not overheated particularly at lower flows and at startup, this smaller total flow area for the partition wall tubes results in an even greater flow velocity through these tubes. This is desirable .in that these tubes are heated from both sides and accordingly subjected to severe operating conditions. With this decrease in total flow area and accordingly increase in flow velocity, adequate cooling of these tubes is provided.

It is to be understood that the invention is not limited to the specific embodiment herein illustrated and described, but may be used in other ways without departure from its spirit and, other various changes can be made which would come within the scope of the invention which is limited only by the appended claims.

I claim:

1. A forced through-flow supercritical vapor generator having a through-flow circuit, an upright furnace with opposed side walls, a gas pass extending laterally from the upper region of the furnace and then downward with side walls parallel with and effectively forming extensions of the furnace side walls, vertically extending tubes lining the walls of the furnace and the gas pass with these tubes forming part of said through-flow circuit, means directing the through-flow up through generally all of the furnace wall tubes in parallel flow relation including those adjacent said gas pass and directly thereafter up through the tubes lining the gas pass, adjacent tubes lining the furnace and the gas pass being integrally joined together throughout the length of the wall with which they are associated and at the juncture of the gas pass with the furnace the adjacent furnace wall and gas pass wall tubes being integrally joined throughout their adjacent length, a recirculation circuit superimposed on a portion of the through-flow circuit including the furnace and gas pass lining tubes and operative to provide a flow therethrough greater than the through-flow at least at and below a predetermined load.

2. A forced through-flow supercritical vapor generator having a elongated furnace with opposed side walls and a combustion gas outlet laterally directed between said walls, a gas pass extending from said outlet and having side walls parallel with the furnace side walls, said furnace having tubes disposed on its walls in side-by-side relation and extending longitudinally thereof with adjacent tubes being welded together throughout the length of the furnace, said gas pass having tubes on its walls in side-byside relation, parallel with the furnace wall tubes, and welded together throughout their overlying length of the gas pass and with the furnace side wall tubes adjacent the gas pass side wall tubes being Welded thereto throughout their adjacent length, a through-flow circuit including the furnace wall and gas pass wall tubes and including means directing the through-flow through generally all of the furnace wall tubes in parallel including those adjacent the gas pass and directly thereafter through the gas pass wall tubes in parallel and in the same direction as the flow through the furnace wall tubes, and a recirculation system superimposed on a portion of the through-flow circuit including the portion comprised of the furnace and gas pass wall tubes and operative to insure that the flow through said portion is at least a predetermined value.

3. In a forced through-flow supercritical vapor generator an upright furnace, a gas pass extending laterally therefrom, said furnace and gas pass having co-planar side walls, the furnace and gas pass walls being lined with single pass vertically disposed tubes in side-by-side relation and welded together to form a fluid-tight wall including the junction of the gas pass with the furnace, at through-flow circuit including means directing the throughflow up through generally all of the furnace wall tubes in parallel including those adjacent the gas pass and directly thereafter up through the gas pass wall tubes in parallel, and a recirculation system superimposed on a portion of the through-flow circuit including the furnace and gas pass wall'tubes operative to insure that tlie flow therethrough does not fall below a given value.

4. A forced through-flow supercritical vapor generator fired by a suitable fuel and including an upright furnace having a pair of opposed, parallel side walls and a laterally directed combustion gas outlet at one end intermediate said side walls, a gas pass connected with the furnace and extending, from said outlet and having side Walls generally co-planar with said furnace side walls and effectively forming extensions thereof, said gas pass including a horizontal portion extending from said outlet and a vertical portion extending from the horizontal portion, vertically extending single pass tubes disposed in side-by-side relation lining the walls of the furnace and gas pass with adjacent tubes being joined together by welding throughout the height of the wall with which they are associated including the juncture of the horizontal gas pass portion with the furnace and with the vertical gas pass portion, headers disposed beneath and adjacent each side wall of the horizontal gas pass portion and with which the inlets of the tubes lining the adjacent wall are connected, header means at the lower end of said vertical gas pass portion with which the inlets of the wall tubes of this portion are connected, a through-flow circuit including the furnace Wall and gas pass wall tubes and having means directing the through-flow up through the furnace wall tubes in parallel and directly thereafter to said headers and said header means and up through the gas pass wall tubes in parallel, and a recirculation system superimposed on a portion of said through-flow circuit including the furnace and gas pass wall tubes and operative to insure that the flow through said portion does not fall below a predetermined value.

5. A forced through-flow supercritical vapor generator I including an upright furnace having a pair of opposed walls and a laterally extending outlet intermediate said walls, a gas pass extending from said outlet and having walls parallel with said opposed walls, a partition wallvertically of said furnace, said partition wall being comprised of and the walls of the furnace and gas pass being lined with vertically extending tubes in side-by-side relation with adjacent tubes being welded together to form a rigid structure throughout their length and including the juncture of the gas pass with the furnace, a through-flow circuit through which the through-flow is forced and including the partition wall, furnace wall and gas pass wall tubes, means directing said through-flow up through the partition w-all tubes in parallel flow relation and directly thereafter up through the furnace wall tubes in parallel flow relation and directly thereafter up through the gas pass wall tubes in parallel flow relation and with the transition zone of the through-flow circuit being in the furnacewall tubes, and a recirculation system superimposed on a portion of the through-flow circuit and connected therewith downstream of the gas pass wall tubes and upstream of the partition wall tubes and operative to insure that the fiow through said portion is of at least a predetermined value.

6. A forced through-flow supercritical vapor generator including an upright furnace having a pair of opposed wal s and l t a y attending outlet intermediate said walls, a gas pass extending from said outlet; and having walls parallel with said opposed walls, a partition wall vertically of'said furnace, said partition wall being comprised of and the walls of the furnace and gas pass being lined with vertically extending tubes in side-by-side relation with adjacent tubes being welded together to forms, rigid structure throughout their length and including the juncture of the gas pass with the furnace, with the total flow area provided by the center wall tubes being substantially less than that provided by the furnace wall tubes, a through-flow circuit throughwhich the through-fiow'is forced'and including the partition wall, furnace wall and gas pass'wall tubes, means directing said through-flow up through the partition wall tubes in parallel flow relation, and directly thereafter up through the furnace wall tubes in parallel flow relation and directly thereafter up through the gas pass wall tubes in parallel flow relation and with the transition zone of the through-flowcircuit being in the furnace wall tubes, and a recirculation system superimposed on a portion of the through-flow circuit and connected therewith downstream of the gas pass Wall tubes and upstream of thepartition wall tubes and. operative to insure that the flow through said portion is of at least a predetermined value.

7. A forced through-flow supercritical vapor generator fired by a suitable fuel and including an upright furnace having a pair of opposed, parallel side walls and a laterally directed combustion gas outlet at one end intermediate said side walls, a gas pass connected with the furnace and extending from said outlet and having side walls generally co-planar with said furnace side walls and effectively forming extensions thereof, said gas pass including a horizontal portion extending from said outlet and a verticalportion extending from the horizontal portion, a partition wall vertically of said furnace, said partition Wall being comprised of and the furnace walls and gas pass Walls being lined with vertically extending single pass tubes disposed in side-by-side relation with adjacent tubes being joined together by welding'throughout the height of the wall withwhich they are associated including the juncture of the horizontal gas pass portion with the furnace andwith the vertical gas pass portion, headers disposed beneath and adjacent each sidewall of the horizontal gas pass portion and with-which the inlets of the tubes lining the 'adjacent wall are connected, header means at the lower end of said vertical gas pass portion with which the inlets of the wall tubes of this portion are connected, a throughflow circuit including the partition wall tubes and the furnace wall and gas pass wall tubes and having means directing the through-flow up through the center wall tubes in parallel, then up through the furnace wall tubes in parallel and then to said headers and header means and upthrough the gas pass wall tubes in parallel with the transition zone being in the furnace wall tubes, and a recirculation system superimposed on a portion of the through-flow circuit including the center, furnace and gas pass wall tubes and operative to increase the flow through said portion over and above the through-flow when the load fallsbelow a predetermined value.

References Cited in the file of this patent UNITED STATES PATENTS 2,840,054 Rowand June 24, 1958 2,987,052 Armacost June 6, 1961 3,003,482 Hamilton et al. Oct. 10, 1961 3,038,453 Armacost June 12, 1962 

1. A FORCED THROUGH-FLOW SUPERCRITICAL VAPOR GENERATOR HAVING A THROUGH-FLOW CIRCUIT, AN UPRIGHT FURNACE WITH OPPOSED SIDE WALLS, A GAS PASS EXTENDING LATERALLY FROM THE UPPER REGION OF THE FURNACE AND THEN DOWNWARD WITH SIDE WALLS PARALLEL WITH AND EFFECTIVELY FORMING EXTENSIONS OF THE FURNACE SIDE WALLS, VERTICALLY EXTENDING TUBES LINING THE WALLS OF THE FURNACE AND THE GAS PASS WITH THESE TUBES FORMING PART OF SAID THROUGH-FLOW CIRCUIT, MEANS DIRECTING THE THROUGH-FLOW UP THROUGH GENERALLY ALL OF THE FURNACE WALL TUBES IN PARELLEL FLOW RELATION INCLUDING THOSE ADJACENT SAID GAS PASS AND DIRECTLY THEREAFTER UP THROUGH THE TUBES LINING THE GAS PASS, ADJACENT THEREAFTER UP THROUGH THE AND THE GAS PASS BEING INTEGRALLY JOINED TOGETHER THROUGHOUT THE LENGTH OF THE WALL WITH WHICH THEY ARE ASSOCIATED AND AT THE JUNCTURE OF THE GAS PASS WITH THE FURNACE THE ADJACENT FURNACE WALL AND GAS PASS WALL TUBES BEING INTEGRALLY JOINED THROUGHOUT THEIR ADJACENT LENGTH, A RECIRCULATION CIRCUIT SUPERIMPOSED ON A PORTION OF THE THROUGH-FLOW CIRCUIT INCLUDING THE FURNACE AND GAS PASS 